1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting data in a wireless Local Area Network (LAN) system. More particularly, the present invention relates to an apparatus and method for preventing collision occurring during data transmission in a wireless LAN system.
2. Description of the Related Art
In general, wireless communication systems can be classified as a mobile communication system, a wireless LAN system, or a Broadband Wireless Access (BWA) communication system. Of these systems, the wireless LAN system has been developed from the wire LAN system with the rapid progress of technology, thereby meeting increasing user demands. The wireless LAN systems are used not only for constructing home networks, but also for constructing small-scale communication networks in companies.
The latest key issue in the wireless LAN system is related to a scheme for improving Quality of Service (QoS). The improvements in QoS are being made in various fields, and one of the fields provides a scheme for decreasing a delay and increasing throughput of the system. However, the throughput of the system cannot be increased by simply increasing only the data rate because of the increase in the number of users using the wireless LAN system. More specifically, the development of technology makes it possible to construct home networks at lower costs and construct small-scale networks in companies. Therefore, the number of stations (or terminals) constituting the networks has increased. Due to the increase in the number of stations, generation of transmission data frequently occurs for each station, causing an increase in the number of cases where different stations simultaneously transmit data.
If multiple stations simultaneously transmit data as stated above, collision occurs between the data transmitted by the stations, raising the problem that access nodes (ANs) or receiving stations of the wireless LAN system cannot normally receive the data. In this case, therefore, the stations may perform retransmission. However, the retransmission causes a reduction in the throughput and occurrence of transmission delay, leading to a decrease in the QoS.
With reference to FIG. 1, a description will now be made of a process of transmitting data using a Medium Access Control (MAC) protocol for a Distributed Coordination Function (DCF) and an Enhanced Distributed Coordination Function (EDCF) in an IEEE 802.11/802.11e system, which is a standard of the current wireless LAN system.
FIG. 1 is a flowchart illustrating a process of transmitting data according to a MAC protocol of the IEEE 802.11 standard.
A MAC protocol processor of a transmitter determines in step 102 whether there is any transmission data in a transmission queue. If it is determined that there is transmission data, the MAC protocol processor generates a backoff time in step 104. The backoff time is generated in the EDCF by unconditionally generating a specific number as a random number in an initial contention window regardless of a state of the network. After generating the backoff time, the transmitter determines in step 106 whether the generated backoff time has a value of ‘0’. If it is determined that the backoff time has a value of ‘0’, the transmitter transmits data stored in the queue, in step 110. However, if the backoff time does not have a value of ‘0’, the transmitter decreases the backoff time in step 108, and returns to step 106 to determine whether the decreased backoff time is ‘0’. That is, the transmitter generates a backoff time using a random number in step 104, sequentially decreases the backoff time generated as a random number through steps 106 and 108, and transmits the data in step 110 if the generated random number is ‘0’.
Thereafter, the transmitter waits for a predetermined time until a response signal is received in reply to the transmitted data in step 112. The transmitter may either successfully receive or fail to receive the response signal from a receiver within the predetermined time as detected in step 114. This is because if the transmitted data suffers collision, the receiver cannot recognize the collision and thus, cannot transmit the response signal. However, if the transmitted data is normally received at the receiver, the receiver transmits the response signal. The receiver transmits only the acknowledge signal (ACK) as the response signal. That is, the receiver transmits the response signal using a CSMA/CA scheme in which the receiver transmits the response signal only when it successfully receives the transmitted data.
Therefore, the transmitter fails to receive the response signal when the receiver suffers reception failure or fails to normally receive the data due to collision. In this case, the transmitter returns to step 104 where it again generates a backoff time in a doubled contention window and then repeats the remaining steps. However, if the receiver normally receives the transmitted data and thus transmits an ACK, the transmitter returns to step 102 where it again determines whether there is any transmission data in the queue.
In the case of transmitting data using the backoff time, a decrease in the number of stations noticeably decreases the probability that collision will occur between backoff times generated as random numbers. However, an increase in the number of stations increases the probability that random numbers generated as backoff times in the stations will be equal to each other. If generation of the same backoff times increases due to the increase in the number of stations, the data throughput decreases in the system according thereto, and the transmission data suffers a corresponding transmission delay, causing a reduction in the QoS.
Accordingly, a need exists for a system and method for preventing collision occurring during data transmission in a wireless LAN system.